The invention relates to thermoplastic particle foams which have cells having a mean cell size in the range from 20 to 500 μm and in which the cell membranes have a nanocellular or fibrous structure having pore or fiber diameters below 1500 nm, and also to processes for producing them.
Expandable polymer mixtures comprising styrene polymers, polyolefins and, if appropriate, solubilizers such as hydrogenated styrene-butadiene block copolymers are known from, for example, DE 24 13 375, DE 24 13 408 or DE 38 14 783. The foams which can be obtained therefrom are said to have better mechanical properties, in particular a better elasticity and a reduced brittleness at low temperatures, and also insensitivity toward solvents such as ethyl acetate and toluene, compared to foams composed of styrene polymers. However, the blowing agent retention capability and the foamability of the expandable polymer mixtures to low densities are not sufficient for processing.
WO 2005/056652 describes particle foam moldings which have a density in the range from 10 to 100 g/l and can be obtained by fusion of prefoamed foam particles produced from expandable, thermoplastic polymer beads. The polymer beads comprise mixtures of styrene polymers and other thermoplastic polymers and can be obtained by melt impregnation and subsequent underwater pelletization under pressure.
Furthermore, elastic particle foams comprising expandable interpolymer particles are known (e.g. US 2004/0152795 A1). The interpolymers can be obtained by polymerization of styrene in the presence of polyolefins in aqueous suspension and form an interpenetrating network of styrene polymers and olefin polymers. However, the blowing agent diffuses rapidly out of the expandable polymer particles, so that they have to be stored at low temperatures and display satisfactory foamability for only a short time.
WO 2005/092959 describes nanoporous polymer foams which can be obtained from multiphase polymer mixtures which comprise blowing agent and have domains in the range from 5 to 200 nm. The domains preferably comprise core-shell particles which can be obtained by emulsion polymerization and in which the solubility of the blowing agent is at least twice as high as in the adjoining phases.
It was an object of the present invention to provide expandable, thermoplastic polymer particles which have a low blowing agent loss and a high expansion capability and can be processed to give particle foams having a high stiffness and at the same time good elasticity, and also a process for producing them.
We have accordingly found the above-described thermoplastic particle foams.
The thermoplastic particle foams preferably have cells having a mean cell size in the range from 50 to 250 μm and a nanocellular structure or a fibrously elongated, disperse phase structure in the cell walls of the thermoplastic particle foams having a mean pore or fiber diameter in the range from 10 to 1000 nm, particularly preferably in the range from 100 to 500 nm.
The polymer matrix of the thermoplastic particle foams preferably comprises a continuous phase which is rich in styrene polymer and a disperse polyolefin-rich phase.
The thermoplastic particle foams particularly preferably comprise a polymer matrix comprising    A) from 45 to 98.9% by weight, in particular from 55 to 89.9% by weight, of a styrene polymer, in particular polystyrene,    B) from 1 to 45% by weight, in particular from 4 to 37% by weight, of a polyolefin, in particular polyethylene, and    C) from 0.1 to 10% by weight, in particular from 3 to 8% by weight, of a hydrogenated or unhydrogenated styrene-butadiene block copolymer.
The thermoplastic particle foams of the invention can be obtained by a process which comprises    a) producing a polymer mixture having a continuous phase and a disperse phase by mixing two incompatible thermoplastic polymers and, if appropriate, a compatibilizer,    b) impregnating this mixture with a blowing agent and pelletizing it to produce expandable thermoplastic polymer particles,    c) prefoaming the expandable, thermoplastic polymer particles to produce foam particles and    d) fusing the prefoamed foam particles in a mold by means of hot air or steam at a processing pressure which is kept sufficiently low for the nanocellular or fibrous structure in the cell membranes to be retained and usually to be in the range from 1.5 to 2.3 bar to produce particle foam moldings.
In a further embodiment, the polymer mixture can, in step b), firstly be pelletized and the pellets can subsequently be after-impregnated with a blowing agent in an aqueous phase under superatmospheric pressure at elevated temperature to produce expandable thermoplastic polymer particles. These can subsequently be isolated after cooling to below the melting point of the polymer matrix or can be obtained directly as prefoamed polymer particles (step c) by depressurization.
It is known from the field of multiphase polymer systems that most polymers are immiscible or only slight miscible with one another (Flory), so that demixing to form the respective phases occurs, depending on temperature, pressure and chemical composition. If incompatible polymers are covalently bonded to one another, demixing does not take place on a macroscopic scale but only on a microscopic scale, i.e. on the length scale of the individual polymer chains. In this case, the phenomenon is referred to as microphase separation. This results in many mesoscopic structures, e.g. lamella, hexagonal, cubic and bicontinuous morphologies which have a strong relationship with lyotropic phases.
The polymer mixture having a continuous phase and a disperse phase can be produced by mixing two incompatible thermoplastic polymers, for example in an extruder.
The polymer mixture preferably comprises from 45 to 98.9% by weight, particularly preferably from 55 to 89.9% by weight, of a thermoplastic polymer A), in particular a styrene polymer such as general purpose polystyrene (GPPS) or high-impact polystyrene (HIPS) or a styrene-acrylonitrile copolymer (SAN) or acrylonitrile-butadiene-styrene copolymer (ABS). Particular preference is given to general purpose polystyrene grades having a mean molecular weight in the range from 120000 to 300000 g/mol and a melt volume rate MVR (200° C./5 kg) in accordance with ISO 113 in the range from 1 to 10 cm3/10 min, for example PS 158K, 168 N or 148 G from BASF Aktiengesellschaft. To improve the fusion of the foam particles during processing to produce moldings, it is possible to add grades which flow readily, for example Empera® 156L (Innovene).
As further component B), the polymer mixture preferably comprises from 1 to 45 percent by weight, in particular from 4 to 37% by weight, of a likewise thermoplastic polymer B) which is incompatible with the thermoplastic polymer A). As polymer B), preference is given to using a polyolefin, e.g. homopolymers or copolymers of ethylene and/or propylene, in particular polyethylene, especially when a styrene polymer is used as polymer A). As polypropylenes, it is possible to use, in particular, injection-molding grades such as Adstif® RA 748 T or impact-modified grades such as Clyrell® EM 2484 from Basell. As polyethylenes, it is possible to use commercially available ethylene homopolymers such as LDPE (injection-molding grades), LLDPE and HDPE or copolymers of ethylene and propylene (e.g. Moplen® RP220 and Moplen® RP320 from Basell), ethylene and oktene (Engage®) or ethylene and vinyl acetate (EVA), polyethylene acrylates (EA) such as Surlyn® grades 1901 and 2601 from DuPont or ethylene-butylene acrylates (EBA) such as Lucofin® 1400 HN, 1400 HM from Lucobit AG. The melt volume index MVI (190° C./2.16 kg) of the polyethylenes is usually in the range from 0.5 to 40 g/10 min and the density is in the range from 0.86 to 0.97 g/cm3, preferably in the range from 0.91 to 0.95 g/cm3. In addition, it is possible to use blends comprising polyisobutene (PIB) (e.g. Oppanol® B150 from BASF Aktiengesellschaft).
As the proportion of polyolefin decreases, the ability to retain blowing agent increases significantly. This significantly improves the storage stability and the processability of the expandable, thermoplastic polymer particles. In the range from 4 to 20% by weight of polyolefin as polymer B), expendable thermoplastic polymer particles having a longer storage life are obtained without the elastic properties of the particle foam produced therefrom being impaired. This is reflected, for example, in a lower compression set εres in the range from 25 to 35%.
To set the desired morphology in a targeted manner, use is usually made of compatibilizers (component C) in amounts of from 0.1 to 10% by weight, preferably from 3 to 8% by weight, based on the polymer matrix.
The compatibilizer leads to improved adhesion between the polyolefin-rich phase and the polystyrene-rich phase and even in small amounts significantly improves the elasticity of the foam compared to conventional EPS foams. Studies on the domain size of the polyolefin-rich phase showed that the compatibilizer stabilizes small droplets by reducing the interfacial tension. An electron micrograph of a section through an expandable polystyrene/polyethylene comprising blowing agent shows disperse polyethylene domains in the polystyrene matrix.
Suitable compatibilizers are, for example, hydrogenated or unhydrogenated styrene-butadiene or styrene-isoprene block copolymers. The total diene content is preferably in the range from 20 to 60% by weight, particularly preferably in the range from 30 to 50% by weight, and the total styrene content is accordingly preferably in the range from 40 to 80% by weight, particularly preferably in the range from 50 to 70% by weight.
Suitable styrene-butadiene block copolymers comprising at least two polystyrene blocks S and at least one styrene-butadiene copolymer block S/B are, for example, star-shaped branched block copolymers as are described in EP-A 0654488.
Further suitable compatibilizers are block copolymers having at least two hard blocks S1 and S2 comprising vinylaromatic monomers together with at least one random soft block B/S comprising vinylaromatic monomers and diene located between them, with the proportion of the hard blocks being above 40% by weight, based on the total block copolymer, and the 1,2-vinyl content of the soft block B/S being below 20%, as are described in WO 00/58380.
Linear styrene-butadiene block copolymers of the general structure S-(S/B)-S which have one or more blocks (S/B)random having a random styrene/butadiene distribution located between the two S blocks are also suitable as compatibilizers. Such block copolymers can be obtained by anionic polymerization in a nonpolar solvent with addition of a polar cosolvent or a potassium salt, as described, for example, in WO 95/35335 or WO 97/40079.
For the purposes of the present invention, the vinyl content is the relative proportion of 1,2 linkages of the diene units, based on the sum of the 1,2, 1,4-cis and 1,4-trans linkages. The 1,2-vinyl content of the styrene-butadiene copolymer block (S/B) is preferably below 20%, in particular in the range from 10 to 18%, particularly preferably in the range from 12 to 16%.
Preference is given to using styrene-butadiene-styrene (SBS) three-block copolymers having a butadiene content of from 20 to 60% by weight, preferably from 30 to 50% by weight, which can be hydrogenated or unhydrogenated, as compatibilizers. These are commercially available, for example, under the trade names Styroflex® 2G66, Styrolux® 3G55, Styroclear® GH62, Kraton® D 1101, Kraton® G 1650, Kraton® D 1155, Tuftec® H1043 or Europren® SOL 6414. These are SBS block copolymers having sharp transitions between B and S blocks. An improvement in the compatibility can additionally be achieved by hydrogenation of the butadiene blocks, e.g. Kraton® G grades.
Furthermore, additives, nucleating agents, plasticizers, flame retardants, soluble and insoluble inorganic and/or organic dyes and pigments, fillers or coblowing agents can be added to the multiphase polymer mixture in amounts which do not impair domain formation and the foam structure resulting therefrom.
As nucleus formers or nucleating agents, it is possible to additionally add, for example, polyolefin waxes or talc in amounts of from 0 to 5% by weight, preferably from 0.5 to 3% by weight, based on the polymers A) to C).
As blowing agent (component D) in step b), preference is given to using from 1 to 15 percent by weight, preferably from 3 to 10 percent by weight, based on the polymer mixture of A) to C), of a physical blowing agent such as C3-C8-hydrocarbons, alcohols, ketones, ethers or halogenated hydrocarbons. Preference is given to using isobutane, n-butane, isopentane, n-pentane or isohexane.
Suitable coblowing agents are ones having a lower selectivity of the solubility in the phase forming the domains, for example gases such as CO2, N2, fluorinated hydrocarbons or noble gases. These are preferably used in amounts of from 0 to 10% by weight, based on the polymer mixture.
Particular preference is given to a continuous process in which a thermoplastic polymer A) which forms the continuous phase, for example polystyrene, is melted in a twin-screw extruder and mixed with a polymer B) which forms the disperse phase and, if appropriate, compatibilizer C) to produce the polymer mixture in step a) and the polymer melt is subsequently conveyed through one or more static and/or dynamic mixing elements and impregnated with the blowing agent in step b). The melt laden with blowing agent can subsequently be extruded through an appropriate die and cut to give foam boards, extrudates or particles.
The melt coming out of the die can also be cut directly by means of underwater pelletization (UWP) to give expandable polymer particles or polymer particles which have been partially foamed to a chosen extent. Setting of a suitable counterpressure and a suitable temperature in the water bath of the UWP thus makes targeted production of foam particles possible.
To produce the expandable polymer particles, the underwater pelletization is generally carried out at pressures in the range from 1.5 to 10 bar. The die plate generally has a plurality of nests having a plurality of holes. At a hole diameter in the range from 0.2 to 1 mm, expandable polymer particles having the preferred average particle diameters in the range from 0.5 to 1.5 mm·0.8 mm are obtained. Expandable polymer particles having a narrow particle size distribution and an average particle diameter in the range from 0.6 to 0.8 mm lead to better filling of automatic molding machines when the moldings have a finely structured shape. Furthermore, a better molding surface having a lower volume of interstices is achieved.
The round or oval particles obtained are preferably foamed to a diameter in the range from 0.2 to 10 mm. Their bulk density is preferably in the range from 10 to 100 g/l.
The mean diameter of the disperse phase of the polymer mixture produced in step a) is preferably in the range from 1 to 2000 nm, particularly preferably in the range from 100 to 1500 nm.
A preferred polymer mixture in step a) is produced by mixing    A) from 45 to 98.9 percent by weight, in particular from 55 to 89.9% by weight, of a styrene polymer, in particular polystyrene,    B) from 1 to 45 percent by weight, in particular from 4 to 37% by weight, of a polyolefin, in particular polyethylene, and    C) from 0.1 to 10 percent by weight, in particular from 3 to 8% by weight, of a hydrogenated or unhydrogenated styrene-butadiene block copolymer.
The invention also provides the expandable, thermoplastic polymer particles which can be obtained as intermediate products in step b) and comprise a polymer matrix comprising    A) from 45 to 98.9 percent by weight, in particular from 55 to 89.9% by weight, of a styrene polymer, in particular polystyrene,    B) from 1 to 45 percent by weight, in particular from 4 to 37% by weight, of a polyolefin, in particular polyethylene, and    C) from 0.1 to 10 percent by weight, in particular from 1 to 8% by weight, of a hydrogenated or unhydrogenated styrene-butadiene block copolymer, with the sum of A) to C) being 100% by weight, and, in addition,    D) from 1 to 15 percent by weight, in particular from 3 to 10% by weight, based on the polymer matrix, of a blowing agent,    E) from 0 to 5, preferably from 0.3 to 3% by weight, of a nucleating agent.
To improve the processability, the finished expandable thermoplastic polymer particles can be coated with glycerol esters, antistatics or anticaking agents.
The fusion of the prefoamed foam beads to produce the molding and the mechanical properties resulting therefrom are improved, in particular, by coating the expandable thermoplastic polymer particles with a glyceryl stearate. Particular preference is given to using a coating comprising from 50 to 100% by weight of glyceryl tristearate (GTS), from 0 to 50% by weight of glyceryl monostearate (GMS) and from 0 to 20% by weight of silica.
The expandable, thermoplastic polymer particles of the invention can be prefoamed by means of hot air or steam to produce foam particles having a density in the range from 8 to 200 kg/m3, preferably in the range from 10 to 50 kg/m3, and subsequently fused in a closed mold to produce foam moldings.